1. Field of the Invention
The present invention relates to a method for correcting a quantity of light emitted from a solid state scanning-type exposing device which is mounted on a digital copying machine, a printer, and the like, and an image forming device provided with the solid state scanning-type exposing device. More specifically, the present invention relates to a method for correcting a quantity of light emitted from an exposing device with high accuracy, and an image forming device capable of providing an image of excellent quality without unevenness of density.
2. Description of the Related Art
As for an image forming device provided with a solid state scanning-type exposing device, it is necessary to prevent each of the light emitting elements of the exposing device from emitting different quantity of light (hereinafter, this state is referred to as unevenness of light quantity). That is, if each of the light emitting elements emits light with different quantity to each other, an output image has unevenness of density. In the image forming device of these kinds, the quantity of light is corrected for each of the light emitting elements so that all the light emitting elements of the exposing device should emit the same quantity of light. The reason why the unevenness of light quantity is generated in the exposing device is that each of the light emitting elements emits light with different quantity to each other, and that the light emitting elements have light emitting characteristics different from each other, and the like.
In a widely used technique for correcting the quantity of light emitted from an exposing device to prevent the unevenness of light quantity from occurring, the light emitting elements thereof are turned on one by one to measure the quantity of light emitted from each of the light emitting elements. From the measured values and the reference value, a value for correcting the light quantity for each light emitting element is calculated. A typical example of such a technique is, for example, disclosed in Japanese Unexamined Patent Publication No. 10-181081. In the method disclosed in this patent publication, a quantity of light emitted from each of the light emitting diodes (LEDs: light emitting elements) is measured. Then, a target value for light quantity correction is determined for each of specified LEDs, based on the quantity of light emitted from each of the LEDs, in such a manner that the specific LEDs have different target values from each other. From the difference between the quantity of light emitted from each LED and the target value for light quantity correction corresponding to the light quantity of the LED, the correction value is calculated. In this method, the target values for light quantity correction are calculated based on the quantity of light emitted from each LED. The target values vary between specified LEDs. Then, the correction value is calculated based on the difference between the quantity of light emitted from each LED, and the target value for light quantity correction corresponding to the light quantity of the LED. By use of the correction value thus calculated, the light quantity is corrected. In this manner, the change rate in quantity of light emitted from the exposing device can be reduced.
Japanese Unexamined Patent Publications Nos. 11-28836 and 10-297015 disclose a technique in which the unevenness of light quantities emitted from light emitting elements of an exposing device is detected by a sensor. Based on the detection result, correction data are produced. By use of the data, the quantity of light to form an image is controlled.
However, including that of Japanese Unexamined Patent Publication No. 10-181081 described above, there is the following disadvantage in the conventional techniques for correcting the quantity of light emitted from an exposing device. In the state where the exposing device is mounted to an image forming device, it is impossible to prevent the unevenness of light quantity from occurring. As a result, the output image includes unevenness of density due to the uneven light quantity of the exposing device. This problem arises because the correction of the quantity of light emitted from the exposing device is conducted based on the quantity of light emitted from each of the light emitting elements thereof when each of them is turned on one by one in an off-line. In other words, the light emission pattern obtained at the time when the light quantity is measured to calculate the correction value is completely different from a light emission pattern obtained at the time when the actual printing is conducted. Due to the difference between the two light emission patterns, the light quantity distribution in the exposing device varies along with the heat generating temperature of the driving IC in the exposing device.
In addition, there are some cases where the distribution of the light quantity in the exposing device varies because the focus position is slightly shifted from the desired position when the exposing device is mounted to the image forming device. Also in such a case, correction of light quantity cannot be conducted properly, and becomes uneven. As a result, the output image includes unevenness of density.